1. Field of the Invention
The present invention relates to an inkjet head that ejects ink toward a recording medium, such as paper, through an ejection orifice.
2. Description of the Related Art
In inkjet heads that perform recording by ejecting ink through ejection orifices, reduction in costs and high-speed printing have been demanded. The cost of an inkjet head can be effectively reduced by reducing the size of the head, in particular, by reducing the size of a recording element substrate. The printing speed can be effectively increased by densely arranging many nozzles in a single recording element substrate.
Photolithography has been developed in which a photosensitive resin is used as the material of a component for forming ejection orifices and flow paths. The surface of the component is selectively exposed to light by using a photo mask, and then a developing process is performed. By using advanced photolithography, it has become possible to form high-precision, high-resolution nozzles. For example, it has become possible to form a nozzle structure described in Japanese Patent Laid-Open No. 2008-49533 by a nozzle forming method using photolithography.
It has been found that the following problems occur when high-speed printing is performed using a head in which nozzles are arranged at a high density.
FIG. 7 is a diagram illustrating an example of a nozzle structure of an inkjet head in which nozzles are arranged at a high density. An ejection amount of nozzle rows C1, M1, Y1, and Y2 is about 5 pl, an ejection amount of nozzle rows C2 and M2 is about 2 pl, and an ejection amount of nozzle rows C3 and M3 is about 1 pl. Here, C, M, and Y represent nozzles that eject cyan ink, magenta ink, and yellow ink, respectively. The arrangement density of the nozzle rows with the ejection amount of 5 pl in an arrangement direction is 600 dpi, and the arrangement density of the nozzles with the ejection amounts of 2 pl and 1 pl in the arrangement direction is 600 dpi. In other words, the nozzles with the ejection amounts of 2 pl and 1 pl are arranged at a density of 1,200 dpi.
A process of recording a high-density solid red image in a single pass was repeated a plurality of times using the above-described inkjet head. More specifically, all of the nozzles in the nozzle rows Y1, Y2, and M1 with the ink ejection amount of 5 pl were used and the image was recorded with the recording density of 600 dpi/75% duty in a carriage scanning direction. All of the nozzles in the inkjet head were subjected to print check immediately after the above-described recording process. As a result, an ejection failure occurred at a plurality of nozzles in the nozzle row M3.
An ejection orifice surface around the nozzle row M3 at which the ejection failure occurred was observed, and it was found that many small ink droplets, that is, so-called ink mist, have adhered to the ejection orifice surface, as shown in FIG. 8. A solid image with a lower image density (for example, image density of 600 dpi/25% duty in the carriage scanning direction) was also printed. In this case, the amount of ink mist adhered to the ejection orifice surface was small, and the ejection failure did not occur.
Thus, high-density images of secondary and tertiary colors were printed using the inkjet head in which a plurality of nozzle rows, which each include densely arranged nozzles, are arranged in a small area. As a result, it was found that a large amount of ink mist adhered to the ejection orifice surface, and the ejection failure was caused by the ink mist.
The above-described problem can be solved by, for example, reducing the print density for a single carriage scanning operation. However, in such a case, the number of times the carriage scanning operation is repeated must be increased to print a high density image. As a result, the printing time increases. In addition, the above-described problem can also be solved by cleaning the ejection orifice surface before the amount of ink mist on the ejection orifice surface reaches a certain amount. However, in this case, the number of times the cleaning process is performed and the time required for cleaning are increased. Therefore, also in this case, the printing time increases.
In light of the above-described situation, an object of the present invention is to provide an inkjet head in which nozzles are densely arranged and which does not easily cause the ejection failure due to the adhesion of ink mist around the ejection orifices even when a high-density image of, for example, a secondary color is printed with a small number of repetitions of the carriage scanning operation.